Here are some suggested settings and methods to get decent results using ReaFir for noise reduction, followed by a workaround that was provided by Alexey Zhelezov on the RWP list. Note that the workaround is only needed if you're running a version of Reaper prior to 6.52, and will likely be removed from this document in time.

Suggested plug-in settings (configure these in the FX window once you've instantiated ReaFir on a track or item):
Set Edit mode to precise, FFT size to 4096, Best quality. Set mode to Subtract.

How to train ReaFir on the noise you want to remove:
1. Find a part of your source material that only contains the troublesome noise. It's important to choose a part of the material that only contains noise, because ReaFir will need to analyze this sound in a latter step. 
2. Wrap the area of noise you've found in a time selection (left and right brackets), hit Home to make sure you're at the beginning of the selection, then toggle Repeat on (Control+R on Windows, Command+R on Mac).
3. In the FX window, enable the "Automatically build noise profile" check box, and hit Space to play the area of noise you've isolated. When you can no longer hear the loop of noise playing, it means that ReaFir has analyzed it and built its noise print, which should only take a couple of seconds. You can stop playback and disable "Automatically build noise profile" when the noise disappears.

The result you'll get with settings configured as suggested above should be reasonable as a starting point. If not, temporarily bypass ReaFir and carefully re-check the area of noise you've chosen to use. Any clicks or other transient sounds contained in that area will likely produce dodgy results. For best results, you'll want to feed ReaFir an example of problematic noise that's uninterrupted, that's why I've recommended using repeat above.

Now, let's do some fine-tuning. There are three approaches to decrease the obviousness of noise reduction. The first two are fully accessible. The final suggestion is a workaround that's no longer needed if your Reaper is up to date, it's just being preserved for folks on versions of Reaper prior to 6.52.

Approach 1: adjust the strength of noise reduction.
This can be done in the OSARA Parameters dialog, which you can get to by hitting P with the track or item where ReaFir has been instantiated selected. Adjust the "EQ Curve Offset" parameter. If you don't have that available, update to the current release of Reaper.

Approach 2: blend some of the unprocessed signal back in.
This can also be done in the OSARA Parameters dialog. Tweak the Wet parameter, which you'll find one up from the bottom of the parameters list. By default, it'll be set to 100% (IE, you're only hearing the wet (processed) result). As you move the slider down below 100%, you'll gradually introduce some of the unprocessed (dry) signal. With some experimentation, you'll hopefully be able to find a balance where noise has been significantly reduced without your audio sounding too butchered. You can A-B the result by changing the Bypass parameter (directly above Wet) to compare how your audio sounds before and after processing.

Workaround for adjusting the strength of noise reduction in older versions of Reaper (over to you, Alexey).
Under Automatically build noise profile option there is big graphical area. Other reference is the middle of the whole window. Use whatever your screen reader support as the reference to move mouse. Once the mouse is in this area, you can click with left mouse button and drag up or down. That will move learned profile, so subtract it more or less from the original signal.
In most cases, moving it slightly lower than default helps make processed signal more natural while still removing most of the noise.

If you've found this cheat sheet helpful and can stretch to buying me a coffee, a humble house in Barcelona or anything in between, my PayPal link is https://paypal.me/ScottChesworth
Updated 29 Oct 2023 by Scott, fixed param names.